Semiconductor integrated circuit device

ABSTRACT

Prior known static random access memory (SRAM) cells are required that a diffusion layer be bent into a key-like shape in order to make electrical contact with a substrate with a P-type well region formed therein, which would result in a decrease in asymmetry leading to occurrence of a problem as to the difficulty in micro-patterning. To avoid this problem, the P-type well region in which an inverter making up an SRAM cell is formed is subdivided into two portions, which are disposed on the opposite sides of an N-type well region NW 1  and are formed so that a diffusion layer forming a transistor has no curvature while causing the layout direction to run in a direction parallel to well boundary lines and bit lines. At intermediate locations of an array, regions for use in supplying power to the substrate are formed in parallel to word lines in such a manner that one regions is provided per group of thirty two memory cell rows or sixty four cell rows.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor integratedcircuit devices and, more particularly, to layout schemes of staticrandom access memory (SRAM) cells. The invention also relates tosemiconductor memory devices using such cells.

One-port SRAM cells with complementary metal oxide semiconductor (CMOS)configurations are typically designed so that each cell consistsessentially of six separate transistors. An exemplary layout of suchcells has been disclosed in, for example, JP-A-10-178110 (laid open onJun. 30, 1998).

In the prior known SRAM cell layout, a semiconductive well region of Ptype conductivity with inverters formed therein is subdivided into twosubregions, which are disposed on the opposite sides of an N-type wellregion while permitting a well boundary line to extend in a directionparallel to bit lines.

The quest for higher integration and ultra-fine patterning techniques inmodern memory devices requires optical exposure apparatus or equipmentto decrease in wave length of beams used therein. To this end, theequipment is designed to employ exposure beams of shorter wavelength,which have advanced from G line to I line, and further to excimer laser.Unfortunately the requirements for micro-patterning architectures growsmore rapidly than technological advance in trend of shorteningwavelengths in such equipment. In recent years, it is strictly requiredthat micropatterning is done with the minimum device-feature length thatshrinks to less than or equal to the wavelength of an exposure beamused. This minimum feature length shrinkage would result in the layoutof IC components—here, memory cells—becoming more complicated in planarshape, which must require the use of irregular polygonal layout patternsincluding key-shaped components, in order to achieve the intendedconfiguration of on-chip circuitry with enhanced accuracy. This makes itimpossible or at least very difficult to microfabricate ultrafine layoutpatterns while disadvantageously serving as the cause of destruction ofthe symmetry of memory cells.

Regrettably the prior art approach is associated with a need to curve orbend a diffusion layer into a complicated key-like shape for the purposeof making electrical contact with a substrate of the P-type well region.Thus, the prior art suffers from a problem as to degradation of thesymmetrization of cell layout pattern, making difficult successfulachievement of microfabrication architectures for higher integrationdensities.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a semiconductordevice is provided which comprises a first inverter including a firstN-channel metal oxide semiconductor (MOS) transistor and a firstP-channel MOS transistor, a second inverter including a second N-channelMOS transistor and a second P-channel MOS transistor with an inputterminal being connected to an output terminal of the first inverter andwith an output terminal being connected to an input terminal of saidfirst inverter, a third N-channel MOS transistor having a sourceconnected to the output terminal of said first inverter and a drainconnected to a first bit line and also a gate connected to a word line,and a fourth N-channel MOS transistor having a source connected to theoutput terminal of said second inverter and a drain connected to asecond bit line plus a gate connected to a word line, wherein the firstand third N-channel MOS transistors are formed in a first P-type wellregion, wherein the diffusion layer has no curved or bent portions whileletting the direction of layout be parallel to the boundary with respectto the first N-well region with the first and second P-channel MOStransistors formed therein, and wherein said second and fourth N-channelMOS transistors are formed in the second P-type well region whosediffusion layer has no bent portions while letting the layout directionbe parallel to the boundary with respect to the first N-well region withthe first and second P-channel MOS transistors formed therein.

The diffusion layer is arranged to have its outer shape that mainlyconsists of straight line segments including the longest straight lineportion which lies parallel to the boundary with respect to the firstn-well region with the first and second P-channel MOS transistors formedtherein, and simultaneously in the case of defining a straight lineacting as the center line extending parallel to such boundary, thelongest line portion is in linear symmetry with such center line; thesecond and fourth N-channel MOS transistors are formed in the secondP-well region whose diffusion layer is mainly arranged by straight linesegments including its longest straight line portion that is parallel tothe boundary with respect to the first n-well region with the first andsecond P-channel MOS transistors formed therein while allowing, whendefining a straight line for use as the center line extending parallelto such boundary, the line portion to be linearly symmetrical with thecenter line. At this time, in the case of employing the linearsymmetrization scheme, complete linear symmetry will not always berequired; alternatively, slight nonsymmetry may also be permissible on acase-by-case basis, which nonsymmetry results from modifying thediffusion layer to have a shape with its portions on the right and leftsides of the center line being substantially the same in area as eachother by way of example.

In accordance with another aspect of this invention, a firstpolycrystalline silicon lead layer for use as the gate of said thirdN-channel MOS transistor and a second polycrystalline silicon lead layerfor use as the gate of said first P-channel MOS transistor and also asthe gate of said first N-channel MOS transistor are disposed in parallelto each other, wherein a third polycrystalline silicon lead layer foruse as the gate of said fourth N-channel MOS transistor and a fourthpolycrystalline silicon lead layer for use as the gate of said secondN-channel MOS transistor and also as the gate of said second P-channelMOS transistor are disposed in parallel to each other, and wherein thefirst and third polycrystalline silicon lead layers are connected via acontact to a second layer of metal lead layer constituting word lines.

In accordance with a further aspect of the invention, the input terminalof said first inverter and the output terminal of said second invertermay be electrically connected together at a contact whereas the inputterminal of said second inverter and the output terminal of said firstinverter are electrically connected together at a contact.

In accordance with yet another further aspect of the invention, a powersupply line connected to the first and second bit lines and the sourcesof said first and second P-channel MOS transistors and a ground lineconnected to the sources of said first and second N-channel MOStransistors may be formed of a third layer of metal lead layer lyingparallel to a diffusion layer. In accordance with a still another aspectof the invention, the first bit line formed of said third layer of metallead layer may be arranged so that it is between a power supply lineformed of said third layer of metal lead layer and a ground line asconnected to the source of said first N-channel MOS transistor formed ofsaid third layer of metal lead layer whereas the second bit line formedof said third layer of metal lead layer is between a power supply lineformed of said third layer of metal lead layer and a ground line asconnected to the source of said second N-channel MOS transistor formedof said third layer of metal lead layer.

In accordance with another further aspect of the invention, the firstand second bit lines and a power supply line connected to the sources ofsaid first and second P-channel MOS transistors may be formed of asecond layer of metal lead layer, wherein word lines are formed of athird layer of metal lead layer, and wherein a ground line connected tothe sources of said first and second N-channel MOS transistors is formedof the third layer and second layer of metal lead layer.

In accordance with a still another further aspect of the invention,memory cells are laid out into the form of an array, wherein contacts toa substrate of P-type well region and a contact to a substrate of N-typewell region are linearly disposed within the array and at upper andlower portions of the array in a direction parallel to the word lines.Although the above is an example which causes two separate P-well to bedisposed on the opposite sides of an N-well region, two N-well regionsmay be disposed on the opposite sides of a p-well region when the needarises.

In accordance with yet another further aspect of the invention, asemiconductor device is provided which comprises a plurality of memoryarrays each including an array of memory cells each having at least apair of N-type well region and P-type well region, and at least oneintermediate region between the memory arrays, wherein the N-type wellregion and P-type well region defines therebetween a boundary with atleast one straight line portion, and wherein a diffusion layer is formedin each of the P-type well region and P-type well region to have aplanar shape of either (1) a shape of rectangle having long sidesextending parallel to said straight line portion or (2) a shaperesulting from letting a plurality of rectangles having long sidesextending parallel to the straight line portion be combined together viarespective short sides thereof; or alternatively,

(1) a shape of rectangle having long sides parallel to said straightline portion or (2) a shape resulting from letting a plurality ofrectangles having long sides parallel to said straight line portion becombined together causing them to extend in the direction of thestraight line.

At least in memory array regions, bit lines are laid out in a directionparallel to the straight line portion whereas word lines are disposed ina direction perpendicular to the straight portion. Preferably, in theintermediate region, at least one type of electrical lead is railed in adirection at right angles to the straight portion, and a lead (e.g.contact) is also formed which is for making electrical contact between apower supply voltage lead and the diffusion layer as formed in theN-well region or P-well region. This lead may include a power supplylead, ground lead, or other potential leads.

The invention is particularly useful for those semiconductor memorydevices having static RAM memory cells each consisting essentially ofsix separate transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an SRAM cell in accordance with Embodiment 1of the present invention, for explanation of a layout pattern of thosecontacts for connection between MOS transistors and those for connectingbetween MOS transistors and metal lead layers.

FIG. 2 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 1 of this invention.

FIG. 3 is a diagram showing a layout of memory cells and theirassociated peripheral circuitry in accordance with Embodiment 2 of theinvention.

FIG. 4 is a diagram showing an SRAM cell in accordance with Embodiment 3of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistor and metal lead layers.

FIG. 5 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 3 of the invention.

FIG. 6 is a diagram showing an SRAM cell in accordance with Embodiment 4of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistors and metal lead layers.

FIG. 7 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 3 of the invention.

FIG. 8 is a diagram showing an SRAM cell in accordance with Embodiment 5of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistors and metal lead layers.

FIG. 9 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 5 of the invention.

FIG. 10 is a diagram showing an SRAM cell in accordance with Embodiment6 of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistors and metal lead layers.

FIG. 11 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 6 of the invention.

FIGS. 12a to 12f are diagrams illustrating in cross-section some ofmajor process steps in the manufacture of the semiconductor device inaccordance with Embodiment 6 of the invention.

FIG. 13 is a diagram showing an SRAM cell in accordance with Embodiment7 of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistors and metal lead layers.

FIG. 14 is a diagram showing a layout of via holes of SRAM cells for usein connecting between multilayered metal leads in accordance withEmbodiment 7 of the invention.

FIG. 15 is a diagram showing an SRAM cell in accordance with Embodiment8 of the invention, for explanation of a layout of those contacts forconnection between MOS transistors and those for connection between MOStransistors and metal lead layers.

FIG. 16 is a diagram showing a layout of via holes of SRAM cells forconnection between multilayered metal leads in accordance withEmbodiment 8 of the invention.

FIG. 17 is a sectional view of a semiconductor device in accordance withEmbodiment 8 of the invention.

FIGS. 18a to 18f are diagrams illustrating in cross-section some ofmajor process steps in the manufacture of a semiconductor device inaccordance with Embodiment 9 of the invention.

FIGS. 19a to 19g are diagrams illustrating in cross-section some ofmajor process steps in the manufacture of a semiconductor device inaccordance with Embodiment 10 of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Several preferred embodiments of the semiconductor memory device inaccordance with the present invention will be explained with referenceto the accompanying drawings below.

Embodiment 1

Referring to FIGS. 1 and 2, there is shown an SRAM cell layout MCembodying the invention. FIG. 1 illustrates well regions and diffusionlayers plus polycrystalline silicon interconnect lead layer as well ascontacts, all of which are formed in or over a semiconductor substratewhereas FIG. 2 depicts a first layer of metal lead layer, via holes 1,second layer of metal lead layer, via holes 2, and a third layer ofmetal lead layer. Symbols used in FIGS. 1 and 2 are indicated at lowerpart of FIG. 2.

An N-channel type MOS transistor TN1 formed in a P-type semiconductivewell region PW1 and a P-channel type MOS transistor TP1 formed in anN-type well region NW1 constitute an inverter INV1. In addition, anN-channel MOS transistor TN2 formed in P-type well region PW2 and aP-channel MOS transistor TP2 formed in N-type well region NW1 constitutean inverter INV2.

An output node of the inverter INV1 is electrically connected by acontact SC1 to an input node of the inverter INV2. An output of theinverter INV2 is electrically connected via a contact SC2 to an input ofthe inverter INV1.

An N-channel MOS transistor TN3 has a drain electrode connected to a bitline BL1, a source electrode connected to a drain of the N-channel MOStransistor TN1, and a gate electrode connected to a word line WD.Similarly an N-channel MOS transistor TN4 has a drain electrodeconnected to a bit line BL2, a source electrode connected to a drain ofthe N-channel MOS transistor TN2, and a gate electrode connected to wordline WD.

The N-channel MOS transistor TN1 and N-channel MOS transistor TN3 areformed over a diffusion layer LN1 whereas the N-channel MOS transistorTN2 and N-channel MOS transistor TN4 are formed over a diffusion layerLN2. The P-channel MOS transistor TP1 is formed over a diffusion layerLP1 whereas the P-channel MOS transistor TP2 is formed over a diffusionlayer LP2.

As the diffusion layers (LN1, LN2, LP1, LP2) are straight lines with nocurved portions, any pattern correction at folded portions is no longernecessary, resulting in the balance between nodes being improved. Incase memory cells are laid out into the form of an array, the diffusionlayers become four separate straight lines extending parallel to the bitlines (BL1, BL2).

In addition, a polycrystalline silicon interconnect lead layer FG3 foruse as the gate electrode of the N-channel MOS transistor TN3 and apolycrystalline silicon lead layer FG4 for use as the gate electrode ofN-channel MOS transistor TN4 are connected to word lines WL which areformed of the second metal lead layer in a vertical direction to the bitlines (BL1, BL2). A polycrystalline silicon interconnect lead layer FG1for use as the gate electrodes of the N-channel MOS transistor TN1 andP-channel MOS transistor TP1 and a polycrystalline silicon interconnectlead layer FG2 for use as the gate electrode of the N-channel MOStransistor TN2 and P-channel MOS transistor TP2 plus the polycrystallinesilicon lead layers (FG3, FG4) are disposed in parallel to the wordlines.

The N-channel MOS transistor TN1 has its source electrode connected to aground potential line Vss1 that is formed of the third layer of metallead layer whereas a source electrode of the N-channel MOS transistorTN2 is connected to a ground line Vss2 as formed of the third layer ofmetal lead layer. In addition, source electrodes of the P-channel MOStransistors (TP1, TP2) are connected to a power supply voltage line Vcc1which is formed of the third layer of metal lead layer.

The bit line BL1 is located midway between the power supply voltage lineVcc1 and ground line Vss1 whereas bit line BL2 is between the supplyvoltage line Vcc1 and ground line Vss2. This structure makes it possibleto reduce cross-couple noises occurring between bit lines, whichadvantageously lowers voltages while increasing operation speeds.

In addition, it is considered that, in case a contact is formed on an n⁻layer through partial cutaway of side spacers during etching of contactholes, a leakage current from the contact via the n⁻ layer to thesubstrate may be produced. When a contact is formed for connectionbetween a polycrystalline silicon lead layer and a diffusion layer, adistance between the diffusion layer LP2 and polycrystalline siliconlead layer FG1 should be greater than the length of a side spacer tothereby eliminate formation an n⁻ layer on the polycrystalline siliconlead layer FG1 side of the diffusion layer LP2, which in turn makes itpossible to prevent a flow of leakage current.

Embodiment 2

Turning to FIG. 3, there is shown an exemplary case where the memorycells MC of Embodiment 1 are laid out into the form of an array. Symbolsused herein are the same as those indicated at lower part of FIG. 2.

The memory cells MC are organized into an array of 256 rows and 128columns, by way of example. In view of the fact that these memory cellsin Embodiment 1 are less in length in the longitudinal direction of bitlines, a total length of such 256 rows of memory cells along the bitlines is shorter than that of prior art devices, thus increasingresultant operation speeds. Neighboring memory cells MC are disposed inlinear symmetry with respect to a “y” axis whereas upper and loweradjacent memory cells MC are in linear symmetry with an “x” axis. Inaddition, specified regions ST for use in supplying more than one powersupply voltage to the substrate are formed at intermediate part of thearray in such a manner that the regions ST extend parallel to word linesWD. One example is that the regions ST are laid out in units of 32-rowgroups. Another example is that regions ST are disposed in units of64-row groups.

An electrical lead Vbn for supplying a voltage potential to the P-wellregions (PW1, PW2) and a lead Vbp for supplying a voltage to the N-wellregion NW1 are formed to lie parallel to word lines. The lead Vbn may becoupled to ground potential Vss or, alternatively, any voltage may beapplied thereto which is potentially different from ground Vss. The leadVbp may be coupled to the power supply voltage Vcc or, alternatively,any voltages potentially different from Vcc may be applied thereto.

Note that in each region ST, a power supply voltage line Vcc forpotentially “reinforcing” a power supply voltage line Vcc1 is formed inparallel to word lines while letting a ground potential line Vss forpotentially reinforcing ground potentials (Vss1, Vss2) is formed inparallel to the word lines.

Also note that the ground lines (Vss1, Vss2) are disposed in a directionperpendicular to the word lines WD whereby upon selecting of a singleword line a voltage potential is supplied from the pair of ground linesto a respective one of those memory cells operatively associated withthis selected word line so that any possible noises occurring at suchvoltage lines are reduced to thereby advantageously speed up an accessoperation while potentially reducing any voltages concerned.

Furthermore, the memory cells MC used are great in width in the wordline direction so that the layout design of sense amplifiers AMP is madeeasier to thereby avoid a need to lay out one sense amplifier for twoadjacent columns of memory cells, which in turn makes it possible topermit one sense amplifier to be laid out at each column. Additionally aword line driver circuit wddrv becomes flat in layout as compared toprior known ones.

Embodiment 3

FIGS. 4 and 5 show a SRAM cell layout MC2 in accordance with Embodiment3. Symbols as used in FIGS. 4-5 are the same as those in FIG. 2. Memorycell MC2 of Embodiment 3 is similar to the memory cell MC of Embodiment1, except that whereas in Embodiment 1 the diffusion layer (LN1, LN2) isformed into a “T”-like planar shape, which resembles a Japanesebattledore plate called “hagoita,” the diffusion layer (LN3, LN4) ofEmbodiment 4 is of a rectangular shape, and that the contacts (SC1, SC2)are replaced with contacts (SC3, SC4) in the first layer of metal leadlayers (M11, M12).

To attain stability, memory cells are typically designed so that thegate width of N-channel MOS transistors (TN1, TN2) is one and a halftimes greater than that of N-channel MOS transistors (TN3, TN4).However, in this case, the shape of diffusion layers resembles a T-likeplanar shape as has been shown in Embodiment 1, which in turn requiresextra techniques including pattern correction procedures such as opticalproximity effect correction (OPC) processes. Additionally this wouldresult in degradation of the balance between transistors. In contrast,Embodiment 3 is such that the diffusion layers (LN3, LN4) are designedto have a rectangular shape whereby the micro-patterning requiredbecomes easier while at the same time enabling improvement in balancebetween transistors. Note however that the resultant gate width ratiobecomes as large as 1.0 time, which in turn requires that the so-calledcell ratio be increased by making different drivabilities therebetween,which is achievable by letting the N-channel MOS transistors (TN3, TN4)be greater in oxide film thickness than N-channel MOS transistors (TN1,TN2), or by increasing the gate length thereof, or alternatively byincreasing the threshold value, or still alternatively by lowering theimpurity concentration of lightly-doped drain regions for relaxation ofelectric fields.

In addition, Embodiment 3 is arranged to employ a contact SC3 and afirst layer of metal lead layer M11 in place of the contact SC1 used inEmbodiment 1 for connection between the output of inverter INV1 and theinput of inverter INV2. With such an arrangement, any curved or bentcontacts are no longer necessary, thereby avoiding the need for patterncorrection (OPC) or the like.

Embodiment 4

FIGS. 6 and 7 show an SRAM cell layout MC3 in accordance with anembodiment 4. Symbols as used in FIGS. 6-7 are the same as those in FIG.2. Memory cell MC3 of Embodiment 4 is similar to the memory cell MC2 ofEmbodiment 3 except that polycrystalline silicon lead layers (FG5, FG6,FG7, FG8) are designed to have a rectangular planar shape. With thiscell, any bent/folded portions are absent thus removing the need for anyadditional pattern correction procedures including OPC processes, whichin turn improves the balance between transistors.

Embodiment 5

FIGS. 8 and 9 show an SRAM cell layout MC4 in accordance with Embodiment5. An explanation on those symbols used in FIGS. 8 and 9 is given atlower part of FIG. 8. Memory cell MC4 of Embodiment 5 is different inlead structure from the memory cell MC of Embodiment 1.

Bit lines (BL3, BL4) and power supply line Vcc2 are formed by use of asecond layer of metal lead layer. A word line WD1 and ground lines(Vss5, Vss6) are formed using a third layer of metal lead layer in aperpendicular direction to the bit lines. Ground lines (Vss3, Vss4) areformed using a fourth layer of metal lead layer in a direction parallelto the bit lines.

A global bit line GB is the electrical interconnect lead that is used incase bit lines are of a hierarchical configuration. The global bit lineGB and bit lines (BL3, BL4) are shielded by the third layer of metallead layer, thus enabling prevention of any possible cross-couplenoises. Additionally the use of ground lines (Vss3, Vss4) makes itpossible to prevent occurrence of cross-couple noises between global bitlines GB.

Embodiment 6

FIGS. 10 and 11 show an SRAM cell layout MC5 in accordance with anembodiment 6. An explanation as to those symbols used in FIGS. 10-11 isgiven at lower part of FIG. 11. Memory cell MC5 of Embodiment 6 isdifferent from the memory cell MC of Embodiment 1 in structure of theso-called three-layered contacts, each of which is for connectionbetween a gate electrode and its associated diffusion layer.

Although in Embodiment 1 a gate electrode is connected to a diffusionlayer via “L”-like contacts SC1, SC2, Embodiment 6 is arranged so thatthe gate electrode is connected to the diffusion layer via silicide inconnect regions SS1, SS2. This makes it unnecessary to bend or curve theindividual contact into the L-like shape in order to connect the gateelectrode to the diffusion layer, which in turn makes it possible toprovide “I”-like rectangular contacts SC5, SC6. No folded/bent portionsare present in the contacts used, which eliminates the need for patterncorrection (OPC).

One practically implementable flow of some major process steps in themanufacture of a device structure employing the connect regions SS1, SS2each for connection between a gate electrode and a diffusion layerassociated therewith via silicide is shown in FIGS. 12a through 12f .Note here that FIGS. 12a-12f are cross-sectional views each indicating aprofile as taken along line A-A′ in FIG. 10, with its right sidecorresponding to the side “A” and with left side corresponding to “A′.”

Fabricate a gate electrode FG made of a chosen polycrystalline siliconmaterial (see FIG. 12a ).

Form a heavily-doped diffusion layer PM of a specified conductivitytype—here, P type (FIG. 12b ).

Form side spacers made of silicon nitride (SiN) by chemical vapordeposition (CVD) techniques, on sidewalls of the resultant gateelectrode FG (FIG. 12c ).

Make use of a resist RG to etch away only one of the SiN side spacerswhich resides on an active region side under a prespecified conditionthat enables etching treatment of a silicon nitride film and oxide filmat increased selectivity (FIG. 12d ).

Fabricate a heavily-doped P (P+) type diffusion layer P+.

After having removed through etching certain part of the oxide film SiOthat lies in the active region that is not covered by any overlying gateelectrode FG, deposit a high-melting-point metal such as refractorymetal including, but not limited to, cobalt (Co); then, anneal theresultant structure to thereby selectively form silicide on thepoly-silicon gate electrode and diffusion layer (FIG. 12f ). At thistime the gate electrode's sidewall and diffusion layer are connectedtogether by such silicide.

Embodiment 7

FIGS. 13 and 14 show an SRAM cell layout MC6 in accordance withEmbodiment 7. An explanation of those symbols used in FIGS. 13-14 is thesame as that given at lower part of FIG. 11. Memory cell MC6 ofEmbodiment 7 is similar to the memory cell MC5 of Embodiment 6 with thecontacts (SC5, SC6) being replaced with contacts (SC7, SC8) in the firstlayer of metal lead layers (M11, M12).

With Embodiment 7, all of the contacts used therein are capable of beingdesigned to have a square planar shape, thus avoiding the need forpattern correction (OPC).

Embodiment 8

FIGS. 15 and 16 show an SRAM cell layout MC7 in accordance withEmbodiment 8. An explanation of those symbols used in FIGS. 15-16 isgiven at lower part of FIG. 16. Memory cell MC7 of Embodiment 8 issimilar to the memory cell MC of Embodiment 1 with the contacts (SC1,SC2) being replaced with local interconnect nodes (LI1, LI2) and alsowith the word lines being modified in such a manner that these areformed in the first layer of metal lead layer rather than in the secondlayer of metal lead layer while also modifying the bit lines and powersupply and ground lines from the third layer of metal lead layer to thesecond layer of metal lead layer. FIG. 17 depicts a sectional view takenalong line A-B of FIGS. 15-16.

Embodiment 1 suffers from limitations as to an inability to dispose thefirst layer of metal leads over the contacts SC1, SC2 due to the factthat these contacts SC1, SC2 are formed of the same layer as theremaining contacts used. In contrast, Embodiment 8 is specificallyarranged to employ the local interconnect nodes LI1, LI2 formed in aseparate layer from the contacts, thus making it possible to dispose thefirst layer of metal lead layer at upper part, which in turn makes itpossible to reduce by one the requisite number of metal lead layers whencompared to Embodiment 1.

Embodiment 9

A process flow of major steps in the manufacture of a three-layercontact section of Embodiment 9 is shown in FIGS. 18a-18f . Thisembodiment 9 is an example of the process for fabrication of thethree-layer contact section as used in Embodiments 1, 3-5 and 8.

Modern LSIs in recent years are typically designed so thatmicropatterning is done to form contact holes by high-selectivityetching techniques with a silicon nitride film or else used as a stopperto ensure that any unwanted over-etching occurs at filed oxide filmseven when contacts are offset in position from diffusion layers and/orgate electrodes due to the presence of possible alignment errors duringphotolithographical patterning processes. In cases gate electrodes areformed to have reduced electrical resistivities by use of the so-calledsalicide processes, it is possible to obtain the intended electricalconduction between a contact lying over a gate electrode and a contactoverlying a diffusion layer even when the both contacts are fabricatedat a time because of the fact that the contact holes required arefabricated after completion of a procedure having the steps of formingsilicide through exposure of selected portions overlying diffusionlayers and gate electrodes after having formed such diffusion layers,depositing thereover a silicon nitride film for use as an etchingstopper, and then further depositing thereover an interlayer dielectricfilm. On the contrary, in the case of either traditionally widelyemployed polycide gate electrodes or polymetal gate electrodes that havebeen developed and announced recently, residual portions of aninsulative film such as oxide film can overlie gate electrodes therebypreventing exposure of these gate electrodes prior to deposition of asilicon nitride film acting as the etch stopper; accordingly, wheneveran attempt is made to form the intended contacts through deposition of asilicon nitride film thereover, the oxide film behaves to partly resideat the bottom of a respective one of those contacts overlying the gateelectrodes, which makes it impossible or at least greatly difficult toprovide electrical conduction required. Embodiment 9 is for enablingachievement of electrical conduction of such contacts overlying gateelectrodes by previous removal of any silicon nitride film portionsoverlying gate electrodes at specified part whereat contact holes willbe defined.

An explanation will now be given of the process flow in the manufactureof Embodiment 9 with reference to FIGS. 18a-18f below.

After having fabricated a gate electrode and a diffusion layer P+,deposit a silicon nitride film SiN for use as an etch stopper (FIG. 18a). The gate electrode is a lamination of polycrystalline silicon PolySiand tungsten W, with an oxide film SiO being further multilayeredthereon as a protective film.

Remove by dry etching techniques specified part of the silicon nitridefilm at locations for definition of a contact hole overlying the gateelectrode (FIG. 18b ).

Deposit a TEOS film and others by plasma CVD methods to thereby form aninterlayer dielectric film (FIG. 18c ).

Let a selected portion of the oxide film at a contact opening portion beetched away by high selective dry etching techniques (FIG. 18d ). Owingto such high selective etching, the silicon nitride film remains freefrom etch treatment and thus acts as a stopper. Since there is nostopper at the portion overlying the gate electrode from which thesilicon nitride film has been removed away in advance, such portion willbe fully etched to the upper part of the gate electrode. This permitselectrical conduction on the gate electrode also.

Remove the silicon nitride film by dry etching techniques (FIG. 18e ).

Deposit a chosen metal such as tungsten in the resulting contact hole,thereby forming a buried plug (FIG. 18f ).

Embodiment 10

Turning to FIGS. 19a-19g , there is shown a process flow in themanufacture of the three-layer contact section of Embodiment 10.Embodiment 10 is one example of the process for forming the three-layercontact section of Embodiments 1, 3-5 and 8.

A difference of the process flow of Embodiment 10 from that ofEmbodiment 9 is that more than one portion of the oxide film at aspecified location whereat a contact hole is to be opened over the gateelectrode has been removed in advance prior to deposition of a siliconnitride film for use as the etch stopper.

The fabrication process flow of Embodiment 10 will be explained withreference to FIGS. 19a-19g below.

Fabricate a gate electrode and a diffusion layer P+(FIG. 19a ). The gateelectrode is a lamination of polycrystalline silicon PolySi and tungstenW, with an oxide film SiO further stacked thereon as a protective film.

Remove by dry etching techniques a specified part of the silicon nitridefilm at the location for definition of a contact hole overlying the gateelectrode, thus letting the gate electrode be exposed at its upper part(FIG. 19b ).

Deposit a silicon nitride film SiN as an etch stopper (FIG. 19c ).

Deposit a TEOS film or else by plasma CVD methods to thereby form aninterlayer dielectric film (FIG. 19d ).

Let a portion of the oxide film at contact opening portion be etchedaway by high selective dry etching techniques (FIG. 19e ). Due to suchhigh selective etching, the silicon nitride film remains free frometching treatment and thus acts as the stopper.

Remove the silicon nitride film by dry etching techniques (FIG. 19f ). Acertain portion from which the oxide film overlying the gate electrodehas been removed prior to deposition of the silicon nitride film is thusexposed at this time, which permits electrical conduction on the gateelectrode also.

Deposit a chosen metal such as tungsten in the resultant contact hole,thereby forming a buried plug (FIG. 19g ).

In accordance with the embodiments stated above, any diffusion layersused therein are specifically designed to have a simplified planar shapeexcluding unnecessarily complicated shapes, which may in turn facilitatemicro-patterning processes.

The invention claimed is:
 1. A semiconductor integrated circuit devicecomprising: a first N-well region provided on a main surface of asemiconductor substrate and extending in a first direction; a firstP-well region and a second P-well region which are formed on the mainsurface of the semiconductor substrate at first and second sidesrespectively of the first N-well region in a plan view, the first andsecond P-well regions extending longitudinally in the first direction,the first N-well region and the first and second P-well regions beingcontinuously extended on a first region and a second region of thesemiconductor substrate in the plan view, the first region and thesecond region being at different positions along the first direction,with the entirety of the second region being disposed to one side of thefirst region in the first direction such that, in the plan view, thesecond region has no overlap with the first region and no portion of thesecond region is adjacent to the first region in a second directionsubstantially perpendicular to the first direction; an SRAM cell formedon the main surface of the semiconductor substrate in the first region,the SRAM cell including: a first inverter comprising a first N-channeltransistor and a first P-channel transistor which are series connectedto each other, a second inverter comprising a second N-channeltransistor and a second P-channel transistor which are series connectedto each other, the first and second inverters being cross-coupled toeach other; a third N-channel transistor as a first transfer transistorcoupled to an output of the first inverter; and a fourth N-channeltransistor as a second transfer transistor coupled to an output of thesecond inverter; a word line formed over the main surface of thesemiconductor substrate and extending longitudinally in the seconddirection in the plan view, the word line being coupled to gateelectrodes of the third and fourth N-channel transistors; and a firstbit line and a second bit line formed over the main surface of thesemiconductor substrate and extending longitudinally in the firstdirection, each of the first and second bit lines being coupled to arespective one of source and drain regions of the third and fourthN-channel transistors; wherein the first and third N-channel transistorshave a first diffusion layer which is formed in the first P-well regionand forms source and drain regions and channel regions, wherein thefirst diffusion layer extends longitudinally along the first directionacross gate electrodes of the first and third N-channel transistors inthe plan view, wherein a first contact for supplying a source potentialto the first N-channel transistor is disposed on the first diffusionlayer at one side of the gate electrode of the first N-channeltransistor, wherein a second contact for providing an electricalconnection between the first bit line and the third N-channel transistoris disposed on the first diffusion layer at one side of the gateelectrode of the third N-channel transistor, and wherein the first andsecond contacts are arranged in a substantially straight line along thefirst direction in the plan view, wherein, in the plan view, no memorycell is formed in the second region, wherein a first well contact forsupplying a first well voltage to the first P-well region is disposed onthe first P-well region at the second region in the plan view, and nowell contact for supplying the first well voltage to the first P-wellregion is disposed on the first P-well region at the first region in theplan view, and wherein the first diffusion layer has no bent portionwith another contact disposed thereon in the plan view.
 2. Asemiconductor integrated circuit device according to claim 1, furthercomprising a second well contact for supplying the first well voltage tothe second P-well region and a third contact for supplying a second wellvoltage to the first N-well region, wherein the second well contact isdisposed on the second P-well region at the second region in the planview, and no well contact for supplying the first well voltage to thesecond P-well region is disposed on the second P-well region at thefirst region in the plan view, and wherein the third well contact isdisposed on the first N-well region at the second region in the planview, and no well contact for supplying the second well voltage to thefirst N-well region is disposed on the first N-well region at the firstregion in the plan view.
 3. A semiconductor integrated circuit deviceaccording to claim 2, wherein each of the gate electrodes of the firstand third N-channel transistors is extended in the second directionwithout bending along the first direction in the plan view.
 4. Asemiconductor integrated circuit device according to claim 3, wherein achannel width of the first N-channel transistor is wider than a channelwidth of the third N-channel transistor, and wherein the channel widthsof the first and third N-channel transistors are defined by a width ofthe first diffusion layer in the second direction.
 5. A semiconductorintegrated circuit device according to claim 1, wherein a configurationof the first diffusion layer in the plan view is defined by a fieldregion formed on the main surface of the semiconductor substrate.
 6. Asemiconductor integrated circuit device according to claim 5, whereinthe field region comprises a field oxide film.
 7. A semiconductorintegrated circuit device according to claim 2, wherein the firstP-channel transistor has a second diffusion layer which is formed in thefirst N-well region and forms source and drain regions and a channelregion, wherein the second diffusion layer extends along the firstdirection across a gate electrode of the first P-channel transistor inthe plan view, wherein the gate electrodes of the first P-channeltransistor and the first N-channel transistor are formed of a firstcommon conductive layer continuously extending over portions of thefirst N-well region and the first P-well region.
 8. A semiconductorintegrated circuit device according to claim 7, wherein the gateelectrodes of the first P-channel transistor and the first N-channeltransistor are formed of a polycrystalline silicon layer.
 9. Asemiconductor integrated circuit device according to claim 2, wherein avoltage level of the first well voltage is lower than that of the secondwell voltage, and wherein the voltage level of the first well voltage issubstantially the same as that of a source potential.
 10. Asemiconductor integrated circuit device comprising: a first N-wellregion provided on a main surface of a semiconductor substrate andextending longitudinally in a first direction; a first P-well region anda second P-well region which are formed on the main surface of thesemiconductor substrate at first and second sides respectively of thefirst N-well region in a plan view, the first and second P-well regionsextending longitudinally in the first direction, the first N-well regionand the first and second P-well regions being continuously extended on afirst region and a second region of the semiconductor substrate in theplan view, the first region and the second region being at differentpositions along the first direction, with the entirety of the secondregion being disposed to one side of the first region in the firstdirection such that, in the plan view, the second region has no overlapwith the first region and no portion of the second region is adjacent tothe first region in a second direction substantially perpendicular tothe first direction; an SRAM cell formed on the main surface of thesemiconductor substrate in the first region, the SRAM cell including: afirst inverter comprising a first N-channel transistor and a firstP-channel transistor which are series connected to each other, a secondinverter comprising a second N-channel transistor and a second P-channeltransistor which are series connected to each other, the first andsecond inverters being cross-coupled to each other; a third N-channeltransistor as a first transfer transistor coupled to an output of thefirst inverter; and a fourth N-channel transistor as a second transfertransistor coupled to an output of the second inverter; a word lineformed over the main surface of the semiconductor substrate andextending longitudinally in the second direction in the plan view, theword line being coupled to gate electrodes of the third and fourthN-channel transistors; and a first bit line and a second bit line formedover the main surface of the semiconductor substrate and extendinglongitudinally in the first direction, each of the first and second bitlines being coupled to a respective one of source and drain regions ofthe third and fourth N-channel transistors; wherein the first and thirdN-channel transistors have a first diffusion layer which is formed inthe first P-well region and forms source and drain regions and channelregions, wherein the first diffusion layer extends longitudinally alongthe first direction across gate electrodes of the first and thirdN-channel transistors in the plan view, wherein a first contact forsupplying a source potential to the first N-channel transistor isdisposed on the first diffusion layer at one side of the gate electrodeof the first N-channel transistor, wherein a second contact forproviding an electrical connection between the first bit line and thethird N-channel transistor is disposed on the first diffusion layer atone side of the gate electrode of the third N-channel transistor, andwherein the first and second contacts are arranged in a substantiallystraight line along the first direction in the plan view, wherein thesecond and fourth N-channel transistors have a second diffusion layerwhich is formed in the second P-well region and forms source and drainregions and channel regions, wherein the second diffusion layer extendslongitudinally along the first direction across gate electrodes of thesecond and fourth N-channel transistors in the plan view, wherein athird contact for providing a source potential to the second N-channeltransistor is disposed on the second diffusion layer at one side of thegate electrode of the second N-channel transistor, wherein a fourthcontact for providing an electrical connection between the second bitline and the fourth N-channel transistor is disposed on the seconddiffusion layer at one side of the gate electrode of the fourthN-channel transistor, and wherein the third and fourth contacts arearranged in a substantially straight line along the first direction inthe plan view, wherein, in the plan view, no memory cell is formed inthe second region, wherein a first well contact and a second wellcontact for supplying a first well voltage to the first and secondP-well regions are disposed on the first and second P-well regionsrespectively at the second region in the plan view, no well contact forsupplying the first well voltage to the first P-well region is disposedon the first P-well region at the first region in the plan view, and nowell contact for supplying the first well voltage to the second P-wellregion is disposed on the second P-well region at the first region inthe plan view, wherein a third well contact for supplying a second wellvoltage to the first N-well region is disposed on the first N-wellregion at the second region in the plan view, and no well contact forsupplying the second well voltage to the first N-well region is disposedon the first N-well region at the first region in the plan view, whereineach of the first and second diffusion layers has no bent portion withanother contact disposed thereon in the plan view, and wherein a voltagelevel of the first well voltage is lower than that of the second wellvoltage, and wherein the voltage level of the first well voltage issubstantially the same as that of a source potential.